repo_zf1/documentation/manual/en/module_specs/Zend_Search_Lucene-BestPractice.xml

<?xml version="1.0" encoding="UTF-8"?>
<!-- Reviewed: no -->
<sect1 id="zend.search.lucene.best-practice">
    <title>Best Practices</title>

    <sect2 id="zend.search.lucene.best-practice.field-names">
        <title>Field names</title>

        <para>
            There are no limitations for field names in <classname>Zend_Search_Lucene</classname>.
        </para>

        <para>
            Nevertheless it's a good idea not to use '<emphasis>id</emphasis>' and
            '<emphasis>score</emphasis>' names to avoid ambiguity in <classname>QueryHit</classname>
            properties names.
        </para>

        <para>
            The <classname>Zend_Search_Lucene_Search_QueryHit</classname> <property>id</property>
            and <property>score</property> properties always refer to internal Lucene document id
            and hit <link linkend="zend.search.lucene.searching.results-scoring">score</link>. If
            the indexed document has the same stored fields, you have to use the
            <methodname>getDocument()</methodname> method to access them:
        </para>

        <programlisting language="php"><![CDATA[
$hits = $index->find($query);

foreach ($hits as $hit) {
    // Get 'title' document field
    $title = $hit->title;

    // Get 'contents' document field
    $contents = $hit->contents;

    // Get internal Lucene document id
    $id = $hit->id;

    // Get query hit score
    $score = $hit->score;

    // Get 'id' document field
    $docId = $hit->getDocument()->id;

    // Get 'score' document field
    $docId = $hit->getDocument()->score;

    // Another way to get 'title' document field
    $title = $hit->getDocument()->title;
}
]]></programlisting>
    </sect2>

    <sect2 id="zend.search.lucene.best-practice.indexing-performance">
        <title>Indexing performance</title>

        <para>
            Indexing performance is a compromise between used resources, indexing time and index
            quality.
        </para>

        <para>
            Index quality is completely determined by number of index segments.
        </para>

        <para>
            Each index segment is entirely independent portion of data. So indexes containing more
            segments need more memory and time for searching.
        </para>

        <para>
            Index optimization is a process of merging several segments into a new one. A fully
            optimized index contains only one segment.
        </para>

        <para>
            Full index optimization may be performed with the <methodname>optimize()</methodname>
            method:
        </para>

        <programlisting language="php"><![CDATA[
$index = Zend_Search_Lucene::open($indexPath);

$index->optimize();
]]></programlisting>

        <para>
            Index optimization works with data streams and doesn't take a lot of memory but does
            require processor resources and time.
        </para>

        <para>
            Lucene index segments are not updatable by their nature (the update operation requires
            the segment file to be completely rewritten). So adding new document(s) to an index
            always generates a new segment. This, in turn, decreases index quality.
        </para>

        <para>
            An index auto-optimization process is performed after each segment generation and
            consists of merging partial segments.
        </para>

        <para>
            There are three options to control the behavior of auto-optimization (see <link
                linkend="zend.search.lucene.index-creation.optimization">Index optimization</link>
            section):

            <itemizedlist>
                <listitem>
                    <para>
                        <emphasis>MaxBufferedDocs</emphasis> is the number of documents that can be
                        buffered in memory before a new segment is generated and written to the hard
                        drive.
                    </para>
                </listitem>

                <listitem>
                    <para>
                        <emphasis>MaxMergeDocs</emphasis> is the maximum number of documents merged
                        by auto-optimization process into a new segment.
                    </para>
                </listitem>

                <listitem>
                    <para>
                        <emphasis>MergeFactor</emphasis> determines how often auto-optimization is
                        performed.
                    </para>
                </listitem>
            </itemizedlist>

            <note>
                <para>
                    All these options are <classname>Zend_Search_Lucene</classname> object
                    properties- not index properties. They affect only current
                    <classname>Zend_Search_Lucene</classname> object behavior and may vary for
                    different scripts.
                </para>
            </note>
        </para>

        <para>
            <emphasis>MaxBufferedDocs</emphasis> doesn't have any effect if you index only one
            document per script execution. On the other hand, it's very important for batch
            indexing. Greater values increase indexing performance, but also require more memory.
        </para>

        <para>
            There is simply no way to calculate the best value for the
            <emphasis>MaxBufferedDocs</emphasis> parameter because it depends on average document
            size, the analyzer in use and allowed memory.
        </para>

        <para>
            A good way to find the right value is to perform several tests with the largest document
            you expect to be added to the index

            <footnote>
                <para>
                    <methodname>memory_get_usage()</methodname> and
                    <methodname>memory_get_peak_usage()</methodname> may be used to control memory
                    usage.
                </para>
            </footnote>

            . It's a best practice not to use more than a half of the allowed memory.
        </para>

        <para>
            <emphasis>MaxMergeDocs</emphasis> limits the segment size (in terms of documents). It
            therefore also limits auto-optimization time by guaranteeing that the
            <methodname>addDocument()</methodname> method is not executed more than a certain number
            of times. This is very important for interactive applications.
        </para>

        <para>
            Lowering the <emphasis>MaxMergeDocs</emphasis> parameter also may improve batch indexing
            performance. Index auto-optimization is an iterative process and is performed from
            bottom up. Small segments are merged into larger segment, which are in turn merged into
            even larger segments and so on. Full index optimization is achieved when only one large
            segment file remains.
        </para>

        <para>
            Small segments generally decrease index quality. Many small segments may also trigger
            the "Too many open files" error determined by OS limitations

            <footnote>
                <para>
                    <classname>Zend_Search_Lucene</classname> keeps each segment file opened to
                    improve search performance.
                </para>
            </footnote>.
        </para>

        <para>
            in general, background index optimization should be performed for interactive indexing
            mode and <emphasis>MaxMergeDocs</emphasis> shouldn't be too low for batch indexing.
        </para>

        <para>
            <emphasis>MergeFactor</emphasis> affects auto-optimization frequency. Lower values
            increase the quality of unoptimized indexes. Larger values increase indexing
            performance, but also increase the number of merged segments. This again may trigger the
            "Too many open files" error.
        </para>

        <para>
            <emphasis>MergeFactor</emphasis> groups index segments by their size:

            <orderedlist>
                <listitem>
                    <para>Not greater than <emphasis>MaxBufferedDocs</emphasis>.</para>
                </listitem>

                <listitem>
                    <para>
                        Greater than <emphasis>MaxBufferedDocs</emphasis>, but not greater than
                        <emphasis>MaxBufferedDocs</emphasis>*<emphasis>MergeFactor</emphasis>.
                    </para>
                </listitem>

                <listitem>
                    <para>
                        Greater than
                        <emphasis>MaxBufferedDocs</emphasis>*<emphasis>MergeFactor</emphasis>, but
                        not greater than
                        <emphasis>MaxBufferedDocs</emphasis>*<emphasis>MergeFactor</emphasis>*<emphasis>MergeFactor</emphasis>.
                    </para>
                </listitem>

                <listitem><para>...</para></listitem>
            </orderedlist>
        </para>

        <para>
            <classname>Zend_Search_Lucene</classname> checks during each
            <methodname>addDocument()</methodname> call to see if merging any segments may move the
            newly created segment into the next group. If yes, then merging is performed.
        </para>

        <para>
            So an index with N groups may contain <emphasis>MaxBufferedDocs</emphasis> +
            (N-1)*<emphasis>MergeFactor</emphasis> segments and contains at least
            <emphasis>MaxBufferedDocs</emphasis>*<emphasis>MergeFactor</emphasis><superscript>(N-1)</superscript>
            documents.
        </para>

        <para>
            This gives good approximation for the number of segments in the index:
        </para>
        <para>
            <emphasis>NumberOfSegments</emphasis> &lt;= <emphasis>MaxBufferedDocs</emphasis> +
            <emphasis>MergeFactor</emphasis>*log
            <subscript><emphasis>MergeFactor</emphasis></subscript>
            (<emphasis>NumberOfDocuments</emphasis>/<emphasis>MaxBufferedDocs</emphasis>)
        </para>

        <para>
            <emphasis>MaxBufferedDocs</emphasis> is determined by allowed memory. This allows for
            the appropriate merge factor to get a reasonable number of segments.
        </para>

        <para>
            Tuning the <emphasis>MergeFactor</emphasis> parameter is more effective for batch
            indexing performance than <emphasis>MaxMergeDocs</emphasis>. But it's also more
            course-grained. So use the estimation above for tuning <emphasis>MergeFactor</emphasis>,
            then play with <emphasis>MaxMergeDocs</emphasis> to get best batch indexing performance.
        </para>
    </sect2>

    <sect2 id="zend.search.lucene.best-practice.shutting-down">
        <title>Index during Shut Down</title>

        <para>
            The <classname>Zend_Search_Lucene</classname> instance performs some work at exit time
            if any documents were added to the index but not written to a new segment.
        </para>

        <para>
            It also may trigger an auto-optimization process.
        </para>

        <para>
            The index object is automatically closed when it, and all returned QueryHit objects, go
            out of scope.
        </para>

        <para>
            If index object is stored in global variable than it's closed only at the end of script
            execution

            <footnote>
                <para>
                    This also may occur if the index or QueryHit instances are referred to in some
                    cyclical data structures, because <acronym>PHP</acronym> garbage collects
                    objects with cyclic references only at the end of script execution.
                </para>
            </footnote>.
        </para>

        <para>
            <acronym>PHP</acronym> exception processing is also shut down at this moment.
        </para>

        <para>
            It doesn't prevent normal index shutdown process, but may prevent accurate error
            diagnostic if any error occurs during shutdown.
        </para>

        <para>
            There are two ways with which you may avoid this problem.
        </para>

        <para>
            The first is to force going out of scope:
        </para>

        <programlisting language="php"><![CDATA[
$index = Zend_Search_Lucene::open($indexPath);

...

unset($index);
]]></programlisting>

        <para>
            And the second is to perform a commit operation before the end of script execution:
        </para>

        <programlisting language="php"><![CDATA[
$index = Zend_Search_Lucene::open($indexPath);

$index->commit();
]]></programlisting>

        <para>
            This possibility is also described in the "<link
                linkend="zend.search.lucene.advanced.static">Advanced. Using index as static
                property</link>" section.
        </para>
    </sect2>

    <sect2 id="zend.search.lucene.best-practice.unique-id">
        <title>Retrieving documents by unique id</title>

        <para>
            It's a common practice to store some unique document id in the index. Examples include
            url, path, or database id.
        </para>

        <para>
            <classname>Zend_Search_Lucene</classname> provides a <methodname>termDocs()</methodname>
            method for retrieving documents containing specified terms.
        </para>

        <para>
            This is more efficient than using the <methodname>find()</methodname> method:
        </para>

        <programlisting language="php"><![CDATA[
// Retrieving documents with find() method using a query string
$query = $idFieldName . ':' . $docId;
$hits  = $index->find($query);
foreach ($hits as $hit) {
    $title    = $hit->title;
    $contents = $hit->contents;
    ...
}
...

// Retrieving documents with find() method using the query API
$term = new Zend_Search_Lucene_Index_Term($docId, $idFieldName);
$query = new Zend_Search_Lucene_Search_Query_Term($term);
$hits  = $index->find($query);
foreach ($hits as $hit) {
    $title    = $hit->title;
    $contents = $hit->contents;
    ...
}

...

// Retrieving documents with termDocs() method
$term = new Zend_Search_Lucene_Index_Term($docId, $idFieldName);
$docIds  = $index->termDocs($term);
foreach ($docIds as $id) {
    $doc = $index->getDocument($id);
    $title    = $doc->title;
    $contents = $doc->contents;
    ...
}
]]></programlisting>
    </sect2>

    <sect2 id="zend.search.lucene.best-practice.memory-usage">
        <title>Memory Usage</title>

        <para>
            <classname>Zend_Search_Lucene</classname> is a relatively memory-intensive module.
        </para>

        <para>
            It uses memory to cache some information and optimize searching and indexing
            performance.
        </para>

        <para>
            The memory required differs for different modes.
        </para>

        <para>
            The terms dictionary index is loaded during the search. It's actually each
            128<superscript>th</superscript>

            <footnote>
                <para>
                    The Lucene file format allows you to configure this number, but
                    <classname>Zend_Search_Lucene</classname> doesn't expose this in its
                    <acronym>API</acronym>. Nevertheless you still have the ability to configure
                    this value if the index is prepared with another Lucene implementation.
                </para>
            </footnote>

            term of the full dictionary.
        </para>

        <para>
            Thus memory usage is increased if you have a high number of unique terms. This may
            happen if you use untokenized phrases as a field values or index a large volume of
            non-text information.
        </para>

        <para>
            An unoptimized index consists of several segments. It also increases memory usage.
            Segments are independent, so each segment contains its own terms dictionary and terms
            dictionary index. If an index consists of <emphasis>N</emphasis> segments it may
            increase memory usage by <emphasis>N</emphasis> times in worst case. Perform index
            optimization to merge all segments into one to avoid such memory consumption.
        </para>

        <para>
            Indexing uses the same memory as searching plus memory for buffering documents. The
            amount of memory used may be managed with <emphasis>MaxBufferedDocs</emphasis>
            parameter.
        </para>

        <para>
            Index optimization (full or partial) uses stream-style data processing and doesn't
            require a lot of memory.
        </para>
    </sect2>

    <sect2 id="zend.search.lucene.best-practice.encoding">
        <title>Encoding</title>

        <para>
            <classname>Zend_Search_Lucene</classname> works with UTF-8 strings internally. So all
            strings returned by <classname>Zend_Search_Lucene</classname> are UTF-8 encoded.
        </para>

        <para>
            You shouldn't be concerned with encoding if you work with pure <acronym>ASCII</acronym>
            data, but you should be careful if this is not the case.
        </para>

        <para>
            Wrong encoding may cause error notices at the encoding conversion time or loss of data.
        </para>

        <para>
            <classname>Zend_Search_Lucene</classname> offers a wide range of encoding possibilities
            for indexed documents and parsed queries.
        </para>

        <para>
            Encoding may be explicitly specified as an optional parameter of field creation methods:
        </para>

        <programlisting language="php"><![CDATA[
$doc = new Zend_Search_Lucene_Document();
$doc->addField(Zend_Search_Lucene_Field::Text('title',
                                              $title,
                                              'iso-8859-1'));
$doc->addField(Zend_Search_Lucene_Field::UnStored('contents',
                                                  $contents,
                                                  'utf-8'));
]]></programlisting>

        <para>
            This is the best way to avoid ambiguity in the encoding used.
        </para>

        <para>
            If optional encoding parameter is omitted, then the current locale is used. The current
            locale may contain character encoding data in addition to the language specification:
        </para>

        <programlisting language="php"><![CDATA[
setlocale(LC_ALL, 'fr_FR');
...

setlocale(LC_ALL, 'de_DE.iso-8859-1');
...

setlocale(LC_ALL, 'ru_RU.UTF-8');
...
]]></programlisting>

        <para>
            The same approach is used to set query string encoding.
        </para>

        <para>
            If encoding is not specified, then the current locale is used to determine the encoding.
        </para>

        <para>
            Encoding may be passed as an optional parameter, if the query is parsed explicitly
            before search:
        </para>

        <programlisting language="php"><![CDATA[
$query =
    Zend_Search_Lucene_Search_QueryParser::parse($queryStr, 'iso-8859-5');
$hits = $index->find($query);
...
]]></programlisting>

        <para>
            The default encoding may also be specified with
            <methodname>setDefaultEncoding()</methodname> method:
        </para>

        <programlisting language="php"><![CDATA[
Zend_Search_Lucene_Search_QueryParser::setDefaultEncoding('iso-8859-1');
$hits = $index->find($queryStr);
...
]]></programlisting>

        <para>
            The empty string implies 'current locale'.
        </para>

        <para>
            If the correct encoding is specified it can be correctly processed by analyzer. The
            actual behavior depends on which analyzer is used. See the <link
                linkend="zend.search.lucene.charset">Character Set</link> documentation section for
            details.
        </para>
    </sect2>

    <sect2 id="zend.search.lucene.best-practice.maintenance">
        <title>Index maintenance</title>

        <para>
            It should be clear that <classname>Zend_Search_Lucene</classname> as well as any other
            Lucene implementation does not comprise a "database".
        </para>

        <para>
            Indexes should not be used for data storage. They do not provide partial backup/restore
            functionality, journaling, logging, transactions and many other features associated with
            database management systems.
        </para>

        <para>
            Nevertheless, <classname>Zend_Search_Lucene</classname> attempts to keep indexes in a
            consistent state at all times.
        </para>

        <para>
            Index backup and restoration should be performed by copying the contents of the index
            folder.
        </para>

        <para>
            If index corruption occurs for any reason, the corrupted index should be restored or
            completely rebuilt.
        </para>

        <para>
            So it's a good idea to backup large indexes and store changelogs to perform manual
            restoration and roll-forward operations if necessary. This practice dramatically reduces
            index restoration time.
        </para>
    </sect2>
</sect1>